I didn't have any problem creating my own cult of Molag Bal and changing my faith. The only problem I had was being excommunicated while 'abusing my power too much'.... I am playing a cult of Cthulhut empire, am the only Cult of Cthulhut empire on the map, am its empress and the religion has no head/leader... This ensured that I couldn't do anything anymore as its impossible to lift excommunication without a figurehead/leader. So yeah, cults are broken, but I didn't have a problem with the event for preparing ground... Now that I think about it, you do know that the cult automatically disbands when you finish preparing ground, right? The whole point of the cult is to convert as many people as possible and then come out as cultist and not need the secret cult anymore.